Came across this as I was doing some RC reading. Thought it would be useful to newbie like me..

Source: http://www.rcuniverse.com/forum/m_3255405/tm.htm

What is the best set-up for my truck? How fast will it go?

These are questions that have haunted me for nearly 10 years. I have been running 1/10 scale R/C trucks off and on, without a real good understanding of how to properly set it up. To me, it was FM ( friggin magic). What pinion should I use? What if I change the spur gear? It was all trial and error that resulted in several melted motors, blown ESC’s and damaged batteries.

But I have found the answer! And it is good!

Question 1: What size spur gear should I run? What about the pinion gear? Well……. When selecting the spur gear and pinion gear size, you need to understand that there is a direct ratio between the tire circumference (referred to as roll-out) and the final drive ratio. That ratio, in most cases, should be as should be as close to 1 to 1 as possible. 1 to 1? What? By a 1 to 1 ratio, I am referring to one revolution of the tire to one revolution of the motor. In doing so, we do our best to maintain the overall efficiency of the motor by keeping it in the ‘Sweet Spot’. Here’s how it works. We’ll use my RC10T3 as the example vehicle. The first thing to consider is the diameter of your tire. It is used to calculate the roll-out of the tire. Multiply the diameter of the tire by pi. ( Ex: 3.25”xpi=10.2101”) Now, you need to consider the final drive ratio of your drive train. Begin by dividing the number of teeth on the spur gear by the number of teeth on the pinion gear. This will give you your external drive ratio. ( Ex: 87/19=4.5789). Now multiply your external drive ratio with your transmission gear ratio ( Ex: 2.4x4.5789=10.9893 ). This is your final drive ratio.

Now, what do we do with these numbers? Subtract the final drive ratio from the roll-out of the tire . (Ex: 10.2101-10.9893= -0.77926) Redo the calculation adding another tooth on the pinion : ( Ex: 87/20= 4.35 ( drive ratio )x2.4 ( transmission gear ratio )= 10.44 ( final drive ratio ) Then subtract the final drive ratio ( 10.44 ) from the roll-out ( 10.2101 ) ( Ex: 10.2101-10.44= -0.230).

So, what is this formula telling us? In order to maintain the 1:1 ratio between your roll-out and your final drive ratio, you need to select a gearing combination that is as close to a 0 margin as possible. In this case, the -0.230 is the optimal choice, since it as close to 0 as we can get, without going over.

How do we know this works? We can check our overall efficiency by dividing the roll-out by the final drive ratio. In this case, the 10.2101 roll-out, and the 10.44:1 final drive ratio means that we have less than a 3% total loss in efficiency. In other words, the tire turns 0.977 times, for every 1 revolution of the motor. Generally, it is best for your motor to gear your vehicle within a + or - 10% margin. ( In the case of this formula, +1 to -1 is an acceptable margin.)

Overgearing a vehicle will add to the speed of the vehicle, but it does so at a tremendous cost. The additional strain placed on the motor by the shorter gearing, will cause tremendous heat build up as the motor struggles to reach its peak RPM. Undergearing a vehicle can be just as damaging. Since the motor will operate at it’s peak RPM almost exclusively, it will accelerate the wear of the components and dramatically shorten it’s service life.

This formula works best with 1/10 scale vehicles running stock to mid-modified motors. ( 27 turn to around 12 turn motors ). However, hotter motors 12 turn and lower may require you to reduce the margin by as much as -1 to combat heat build-up. But never gear your vehicle below an overall efficiency of 90%.

We hav simplified this equasion, by adding a Gear Wizard Calculator. All you need to do is add the right entries, and the calculator does the math for you!

http://www.rcuniverse.com/community/gearwizard.cfm

Question 2: How fast will it go? Well, we have half the equation already. Using the circumference of the tire divide that by the final drive ratio. ( Ex: 10.2101/10.44=.977797) multiply that number by the maximum working rpm that your motor is capable of. Most motors are rated at XXX RPM, XXX rpm/volt, or XXXk/v. With the peak RPM rating, simply use that number, unless you are using a higher voltage cell. For RPM/v (k/v) you simply multiply its number of RPM/volt by the number of volts supplied to the motor. I have a Trinity Jade 15 turn motor. It is rated for 28,500 rpm. @ 7.2 volts. ( Ex: .977797*28500= 27,867.2145 inches per minute ) Convert that sum to feet per minute by dividing by 12 ( 12 inches in a foot) ( Ex: 27,867.2145 /12=2322.2678 feet per minute ). Now multiply your feet per minute by 60 minutes ( Ex: 2322.2678 *60= 139,336.0725 feet per hour ). Now divide your feet per hour by 5280 ( the number of feet in a mile ). ( Ex: 2322.2678 /5280= 26.389 miles per hour ). Keep in mind that this number is entirely theoretical and is affected by the age of your motor, condition and charge of your battery, friction and or slip from your tires etc. Despite this, it is a pretty good estimate of just how fast you can go with a given motor!

So, the key to speed and longevity is a high rpm motor coupled to a properly geared drive train. It will make for many a happy afternoon of backyard bashing with your truck!